Implement persisted light/dark theme toggle in Settings

[Lakes41]

Difficulty: Intermediate
Type: feature

Background
The Settings page (/settings) is described as holding app settings, but there's no evidence of a theme toggle despite Tailwind CSS being the styling layer, which supports dark mode out of the box.

Problem
Users cannot switch between light and dark themes, and any preference isn't remembered across sessions.

Expected outcome
A theme toggle in Settings switches the whole dashboard between light and dark mode, and the choice persists across reloads.

Suggested implementation

• Enable Tailwind's darkMode: 'class' strategy in the Tailwind config.
• Add a small ThemeProvider (context) wrapping the app layout that reads/writes the preference.
• Persist preference via a cookie or localStorage (guard SSR access appropriately in Next.js 14).
• Add a toggle UI control in the Settings page.

Acceptance criteria

• Toggling updates all pages immediately, not just Settings
• Preference persists after a full page reload
• No flash-of-wrong-theme on initial load
• Works with existing Tailwind utility classes without a full re-theme

Likely affected files/directories

• apps/dashboard/app/settings/
• apps/dashboard/tailwind.config.*
• apps/dashboard/app/layout.tsx

[Leyrioh7]

Hi Maintainer,

I'd like to work on this issue, I have experience working on this and can submit a PR within 48 hours

[grantfox-oss]

🦊 GrantFox — @Leyrioh7 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #289)
2. Your PR will be reviewed by the Adamantine-guild maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Leyrioh7's PR #309 was approved and merged by @Lakes41.

🏆 @Leyrioh7: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (83 total points). Track your full progress on GrantFox.

👏 Great work, @Leyrioh7! Keep contributing to Adamantine-guild.